The present invention relates to an automatic diaphragm control device for use with a CCTV camera which has recently found many applications.
In the case where a video signal supplied from a television camera is utilized for control of the objective diaphragm incorporated in this camera, one of the essential measures to obtain the optimum picture is a sensitivity adjustment to control absolute brightness of an object to be picked up. Light and shade ratio of different areas in the same object, i.e., contrast, is also the important factor to be considered. In the natural world, for example, the contrast lies within a range of tens:one even in cloudy daylight and within a range of hundreds:one in cloudless daylight. In the picture reproduced by a television, on the contrary, the contrast lies within a range as narrow as of 30 to 40:1. For a high contrast as in the natural world, therefore, it is not adequately useful to rely upon said sensitivity adjustment alone in order to obtain the optimum picture and it is necessary to control a proportion of the area in one and the same object corresponding to a bright portion of the video signal coming from the camera and to utilize this video signal as a diaphragm control signal. Also for the object, for example, in the natural world, in which not only the contrast but also the other factors continuously vary over a wide range, it is desired and has been seriously demanded that means for said control can be continuously regulated at will at a place adjacent to or remote from the camera objective to maintain the optimum picture.
It has also been demanded, in the CCTV system widely adopted in many fields for various applications, to develop a diaphragm control device for CCTV camera of high reliability permitting said automatic control means to be easily remote-manipulated, if desired, to make said system perfect. However, it will be impossible to achieve such diaphragm control for the CCTV camera of so many types with a high stability unless such diaphragm control can be stabilized within an adequately wide range of source voltage.